Speaking of Content-Location, I see w3m-region in
mm-inline-text-html-render-with-w3m can be passed a url location so
relative links and images go to the right place.  I wonder if something
like below could pass that through from RFC2557 mhtml parts.

The decode bit is fairly nasty, because I believe in a multipart it's
necessary to propagate the Content-Location down into subparts (if they
don't have their own Content-Location).  Or at least that's my reading
of the rfc and the sample messages at
http://people.dsv.su.se/~jpalme/ietf/mhtml-test/mhtml.html

On those samples you can see the relative links for the images come out
right with w3m-region passed the Content-Location urls.  (Looking just
with describe-char if you've got image display disabled, as I do, and
without worrying, yet, whether w3m can pick out the image contents from
the multipart/related parts instead of downloading ...)